Recently, as electronic equipment is reduced in size and weight, high-frequency capacitors with a lower impedance in a high frequency region, a smaller size, and a larger capacity have been required.
Although mica capacitors, film capacitors, ceramic capacitors, and the like are used as high-frequency capacitors, these capacitors are not suitable for large capacity applications.
On the other hand, examples of capacitors suitable for large capacity applications include aluminum electrolytic capacitors, tantalum electrolytic capacitors, and the like. However, although the aluminum electrolytic capacitor can achieve a large capacity with low cost, it has problems such as temporal change due to evaporation of an electrolytic solution as it uses the electrolytic solution, high impedance in a high frequency region, and the like.
The tantalum solid electrolytic capacitor is a capacitor with less capacity degradation, as it uses solid manganese dioxide as an electrolyte. However, since a coating film of manganese dioxide has poor self-repairing ability, there are disadvantages such as the risk of catching fire when a dielectric coating film is damaged while electric power is being supplied.
Consequently, to solve the problems described above, it has been proposed recently to use a conductive polymer that is excellent in electric conductivity and easily forms a solid electrolyte, as a solid electrolyte. With this technique, it has become possible to obtain a solid electrolytic capacitor that can be manufactured with less cost, ensures capacitance, and has no damage in a dielectric coating film and less leakage current, when compared with the solid electrolytic capacitor described above.
Herein, the conductive polymer refers to a polymer obtained by polymerizing pyrrole, thiophene, furan, aniline, and the like.
Such a solid electrolytic capacitor has also been required to have a reduced ESR (Equivalent Series Resistance), a reduced LC (Leakage Current), and the like to improve reliability.
To solve the problems as described above, methods of forming a conductive polymer as a solid electrolyte layer, using an electrolytic polymerization liquid prepared by mixing a plurality of dopant-introducing agents having different properties, have been proposed (for example, Patent Document 1).    Patent Document 1: Japanese Patent Laying-Open No. 2005-116777